The present invention relates to the the wire end section of a paper making machine, and more particularly to the cooperation between the head boxes and the wire belt or wire at the wire end section.
That section of the machine has a wire belt or wire, usually in the form of an endless belt that is driven and guided over guide rollers. There is at least one head box for supplying pulp suspension to the wire. The head box includes a channel that is defined between two guide walls, one wall more upstream in the direction of movement of the wire and one more downstream, and the channel terminates in an outlet opening through which a stream of pulp suspension is fed to the wire. One of the two flow guide walls, and particularly the downstream one, has an extension at its end closer to the wire which defines a convexly curved slide shoe which the wire moves past. The curved slide shoe cooperates with the wire passing it to define a web forming zone through which the pulp is moved. The pulp suspension is drained of water through the wire.
One such arrangement is known from German Provisonal Patent Auslegeschrift 29 08 791 published Dec. 18, 1980, which corresponds to U.S. Pat. No. 4,308,097. Before the endless wire belt, which carries the web of fiber that is being formed, travels over the curved continuation of the first flow-guide wall of the head box, it is guided over the outside of the other flow-guide wall of the head box.
One disadvantage of this arrangement is that the channel which is defined by the two flow-guide walls must have a relatively sharp deflection in the vicinity of the outlet opening. Stated in other words, the central flow thread of the stream of pulp has a smaller radius of curvature in the region of the outlet opening from the head box than in the web-forming zone. As a result, there is a danger that with high operating speeds, secondary flows will be produced in the web-forming zone which will move transversely to the direction of the main suspension flow, as seen in longitudinal section. These secondary flows can lead to a non-homogeneous distribution of the fibers. Furthermore, in extreme cases, cavitation phenomena occur.
Another disadvantage is that if a plurality of head boxes, arranged one behind the other, are associated with the wire belt for producing a multi-ply web, it is necessary to transfer the first layer of the fiber web onto a felt belt after that fiber layer has been formed. The wire belt must thereafter be led to the second head box, whereupon a layer of the fiber web formed there must again be transferred to the felt belt. This may be repeated several times. In this case, the head boxes must be arranged in a very narrow spaces between the wire belt on the one hand and the felt belt on the other hand. Therefore, as a rule, it is necessary to refrain from using the well-proven nozzle-like head box construction and to instead provide a "folded" flow channel.
The object of the present invention is further to develop the known arrangement so that a fiber web of high quality, i.e with a homogeneous distribution of the fibers, can be produced, even with extremely high speeds of travel of the wire.
This object is achieved by the slide shoe being located beyond the outlet opening of the head box toward the wire belt and downstream of the outlet opening with respect to the movement of the wire belt. The slide shoe includes a generally convexly curved first guide surface, which curves gradually from being oriented more transversely to the path of the wire belt past the head box at the outlet opening to being oriented more parallel to that path nearer the belt and further downstream from the outlet opening in the movement of the belt. The first guide surface cooperates with the belt to define a web-forming zone in which a web of the pulp suspension becomes formed. The radius of curvature of the convex first guide surface changes. In the region of the outlet opening from the head box, this radius is at least as large as the radius of curvature of the guide surface further downstream along the path of the wire belt, and preferably larger.
Additionally, there is a supporting device at the opposite surface of the wire belt from the head box and this supporting device includes its own convexly curved second guide surface for engaging the opposite surface of the wire belt. The second guide surface is also curved gradually, from being more parallel to the path of the wire belt to being more transverse to that path downstream in the path of the wire belt. The second guide surface is located in the vicinity of the outlet opening, but upstream of the first guide surface in the path of the wire belt, so as to introduce the wire belt into the web-forming zone. With the second guide surface, the wire belt is no longer supported by the outer side of the upstream one of the two flow-guide walls of the head box, for introducing the wire belt into the web-forming zone. Instead, the supporting device, with its convexly curved wire support surface, is arranged for this purpose within the loop of the wire belt. This has two benefits. Sufficient space is obtained in the vicinity of the outlet opening of the head box to assure the stable construction of the flow guide walls of the box, without the channel defined by the guide walls having to be substantially curved. The flow of fibrous pulp suspension can preferably be guided even without curvature and therefore substantially linearly. In this way, it is possible to avoid transverse flows, which would disturb the homogeneity of the web of fibers being formed.
It is now also possible to introduce the wire belt, together with a web of fibers previously formed on the wire belt, into the web-forming zone of an additional head box. In this way, as is known from other multi-ply paper making machines (German Unexamined Application for Patent Offenlegungsschrift 25 52 485 published June 2, 1977, FIG. 4), a second layer of a fiber web can be formed directly on the web layer which is already present on the wire belt. As is known, the first layer of fiber web, which is alredy present on the wire belt, serves as a filter-aid layer during removal of water from the additional layer through the wire belt, so that fewer fines and fillers are discharged together with the drainage water or backwater. In addition, there is no longer any space limits due to a felt belt having to cooperate with the arrangement of the head boxes. Therefore, use can be made, for instance, of well-proven nozzle head boxes.
German Provisional Patent Auslegeschrift No. 19 31 686 published Feb. 26, 1970, corresponding to U.S. Pat. No. 3,582,467, shows a drainage box, having a convexly curved wire guide surface and the drainage box is located within a wire belt loop, which is within the region of the outlet opening of a head box. The drainage box is swingable toward the outlet opening of the head box or away from it.
The drainage box has a plurality of bars extending transversly to the direction of travel of the wire. Drainage slits are present between these bars. However, such a drainage box has the disadvantage, particularly when it is arranged directly at the beginning of the web-forming zone, that it causes a non-uniform distribution of the fibers in the web of paper and a high loss of fines and fillers. Furthermore, the aforesaid German patent concerns a two-wire paper making machine. There is a high structural expense for providing two wire belts. Furthermore, a far greater amount of energy is required for driving two wire belts and a greater expense is incurred for cleaning them. In many two-wire paper making machines, the formation of the web is also disturbed because there is a long free jet of pulp between the outlet opening of the head box and the web-forming zone. To avoid this disadvantage, according to German Provisional Patent Auslegeschrift No. 1 931 686, it is necessary to provide flexible flow guide walls, which rest against the wire belts. According to the present invention, this disadvantage is avoided from the outset in that the jet of pulp is guided in any event on one side, without interruption, by the extended flow-guide wall or slide shoe.
On the other or upstream side of the jet of pulp at the outlet opening, the free length of the wire belt can be kept particularly short by developing the wire belt supporting device that is within the wire-belt loop with its second guide surface having a radius of curvature which is less than 200 mm. and preferably even less than 100 mm., and so that the radius of curvature of the second guide surface of the wire supporting device is smaller than the means radius of curvature of the first guide surface of the slide shoe, which is at the opposite side of the wire belt and at the downstream side of the outlet opening. The wire belt supporting device with the convexly curved second guide surface comprises a simple, solid wire supporting rail which is free of drainage slits and has a radius of curvature which can be selected particularly small. As compared with a rotating roller, this construction has the advantage that it is not necessary to take critical speeds of rotation into consideration. The supporting device can therefore be shaped entirely independently of the speed of the machine.
In order to be able to counteract possible inaccuracies in the manufacture of the wire supporting rail inside the loop of the wire belt or of the adjacent flow guide wall of the head box, the wire supporting rail is displaceable as a whole transversely to the stream of pulp, in the direction toward the outlet opening of the head box or back. In addition, the path of displacement of this rail over the width of the wire belt can be preferably set at different values. For this purpose, a plurality of individually adjustable threaded spindles can be provided, for instance, over the width of the rail.
When the wire belt enters the web-forming zone, air may be introduced into the stream of pulp. This air disturbs the web-forming process. This danger is counteracted by providing the wire support rail that is typically within the wire belt loop with a downstream runoff line which is located upstream along the path of movement of the wire belt from the place along the path of the belt where the stream of pulp exiting from the channel at the head box impinges on the belt. The air present in the meshes of the wire belt, behind the line at which the wire belt runs off from the wire belt support rail, can escape from the meshes into the inside of the wire belt loop upon the impingement of the jet of pulp against the wire belt.
French Unexamined Application for Pat. No. 2 457 340 published Dec. 9, 1980 (W080/02575 published Nov. 27, 1982 as an International Application under PCT) shows a wire end section of a paper making machine in which a wire belt is guided in the web-forming zone over a suction box. The suction box has a wire support rail only at the inlet end and at the outlet end. The vacuum in the suction box causes strong bending of the wire belt. In the web-forming zone, the stream of pulp is covered by a flexible lip, which is fastened to the upper flow-guide wall of the head box. This lip bends correspondingly to the bending of the wire belt. One disadvantage of this known construction is that a large amount of energy is required to produce the vacuum in the suction box. Another disadvantage is that the flexible lip can enter into oscillation. This generally produces large variations in the weight per unit of surface of the paper web produced. Furthermore, there is a danger of paper web breakage. In addition, the course of a curvature of the web-forming zone can be controlled at most by changing the tension of the wire or the vacuum. The invention, on the other hand, makes it possible for a given course of the curvature to be precisely determined at the stationary slide shoe that is downstream of the head box, both in the direction of travel of the wire belt, and also transversely thereto, for instance at the edges.
The curved slide shoe of the invention can be developed as a rigid extension of the associated flow-guide wall. However, the slide shoe is preferably transversely displaceable to the flow of pulp. Preferably, there is a sealing surface which is located between the immovable, downstream flow guide wall and the relatively displaceable slide shoe. If necessary, the slide shoe can be moved to extend slightly into the pulp channel. This enables the size of the outlet opening to be adjusted in order to change the quantity of pulp that emerges. The slide shoe is adjustable so that its path of displacement over the width of the wire belt is adjustable to different extents. As a result, the stream of pulp can be made uniform over the width of the machine. Placing the sealing surface, which cooperates with the slide shoe, in front of the outlet opening, with respect to the direction of flow of the pulp suspension helps to stabilize the stream of pulp in the outlet opening.
Because the radius of curvature of the slide shoe increases in the direction of flow of pulp, it is possible to take into account the fact that the removal of the water takes place faster at the beginning of the web-forming zone than at the downstream end. In this case, therefore, the wire belt is of approximately constant curvature in the drainage zone. However, it may also be advisable, particularly in the case where high homogeneity is required of the sheet to be formed, to cause the curvature of the slide shoe in the direction of flow to decrease for decreasing the pressure in the direction of flow, which compensates for the friction on the slide shoe. The mean radius of curvature of the slide shoe is generally between 100 and 800 mm and preferably between 150 and 300 mm.
In order precisely to define the end of the drainage and web forming zine, the wire belt should preferably be deflected slightly, at most by 5.degree., by a run-off edge provided at the end of the slide shoe, i.e. it is deflected from being tangent to the slide shoe at the run-off edge.
For furthr removal of water from the fiber web being formed, a pressure chamber is developed downstream of the slide shoe, with respect to the path of the wire belt and upstream of throttle means which are located near, but out of contact with, the wire belt. The air pressure in this chamber should in general be between 5,000 and 10,000 pascals, and preferably between 2,000 and 7,000 pascals.
If a multi-ply sheet of fibers is to be produced, several, for instance two, head boxes are arranged behind one another at the same surface of the wire belt in such a manner that at least one second layer of fiber web produced from the second head box is formed on a first layer of the fiber web produced from the first head box. As already mentioned above, the first layer of fiber web serves here as a filter-aid layer for the removal of water from the second fiber web layer. By this process, there is a higher retention of fines and fillers. Each of the head boxes has a respective slide shoe at its downstream flow guide wall. The length of the arc of that slide shoe, measured in the direction of flow and of movement of the wire belt, is larger for each succeeding head box in the direction of travel of the wire belt. Correspondingly, the mean radius of curvature of each slide shoe becomes progressively smaller with each succeeding head box.
Higher retention of fines and fillers upon the drainage of the first layer of fiber web is a further object of the invention. To achieve this, there is a separate drainage water or backwater recovery system for each head box. The drainage water from each of the head boxes is collected in a respective collection reservoir. The drainage water from the first head box is pumped to the pulp dispensing channel of the second head box, while the drainage water from the second head box is pumped to the pulp dispensing channel of the first head box. With a wire end section having two head boxes, the drainage or backwater coming from the first head box and having a relatively high proportion of fines and fillers, is used to dilute the pulp for the second head box. In this way, a large part of the fines and fillers, which are "lost" after drainage of water from the pulp from the first head box, still pass into the paper web which is being formed. In constrast, in known paper making machines, there is a frequent problem in that the content of fines and fillers in the backwater of the first head box gradually becomes so great, particularly with certain types of waste paper as raw material, that desired quality of paper can no longer be obtained. A larger proportion of the backwater than should otherwise be necessary must then be withdrawn from the circuit. As a result, the filter system of the conventional paper factory is subjected to a heavier load. All of these disadvantages can be eliminted with the above described features. This is best done if, in the above-indicated example with two head boxes, approximately equal amounts of backwater are produced in the two web-forming zones. Therefore, provision should be made for any excess backwater to be able to flow from the one backwater collection system into the other.
Illustrative embodiments of the invention will be described below with reference to the accompanying drawings.